This invention relates to a method of cooling a gas turbine blade and a cooling structure therefor. More particularly, it relates to a cooling method and a cooling structure of the open circuit type which exploits liquid in a moving blade of a high-temperature gas turbine.
The thermal efficiencies of gas turbines are enhanced by raising the temperature or pressure of a gas at a turbine inlet. In gas turbines presently put into practical use, the system of cooling blades with air is adopted, and the operating temperature is deemed to be limited to 1,100.degree.-1,200.degree. C. In order to operate the gas turbine at a still higher temperature, it is necessary to employ a coolant of greater cooling effect (in general, water). Although a blade cooling method employing water as the coolant has not been put into practical use yet, several proposals have been made. An example is described in Japanese Patent Application Publication No. 48-25441. In this example, water is introduced from outside a turbine casing into a nozzle which is disposed in a manner to extend towards the root part of a moving blade fitted in the outer periphery of a disc, the water is injected from the nozzle to the root part of the moving blade, and the injected water passes through coolant passageways provided in the moving blade and is emitted from the tip of the moving blade. Although such a cooling method is advantageous in being good in the heat transfer performance and great in the cooling effect, it has various problems. More specifically, the root part of the moving blade and a casing part surrounding the moving blade are eroded and damaged by the water drops injected from the nozzle and by the unvaporized water drops emitted from the tip of the moving blade, respectively. In case where any excess liquid has been supplied, the unvaporized liquid emitted from the tip of the moving blade deprives the high-temperature operating gas of large amounts of heat for vaporization, to lower the temperature of the operating gas and to degrade the output of the gas turbine. On the other hand, in case where the supply of the water has stopped even temporarily, the temperature of the blade rises suddenly because the coolant consists only of the water supplied from the nozzle. Then, the blade will break due to an insuficient strength, or the operation of the turbine cannot but be stopped. Furthermore, since the blade is at a high temperature, it is liable to film boiling. In order to prevent the film boiling, large quantities of water must be caused to flow through the coolant passageways in the blade at high speed. This results in emitting the unvaporized liquid drops as stated above, and brings about the problem of erosion.
Besides, there has been known a system wherein, as disclosed in Japanese Patent Application Laying-open No. 50-73012, air and water are introduced into the interior of a blade by separate conduits and are injected with their nozzle portions arranged so as to oppose to the inner surface of the blade to be cooled. In this system, the distance from the coolant injecting ports to the inner surface of the blade being the surface to-be-cooled is short, and a homogeneous atomized coolant is difficult to be obtained. Moreover, the air and the liquid have different specific gravities. Especially in a centrifugal field, the liquid of greater specific gravity is prone to be pushed in the direction opposite to the rotating direction of the blade and flow separately from the air on account of Coriolis's force. The liquid strikes and cools the portion to-be-cooled, and the cooling effect varies greatly between the suction and pressure sides of the blade. Therefore, a sharp temperature gradient arises in the material of the blade. This leads to increase in the thermal stress, and has the possibility of shortening the lifetime of the blade. Since, in this system, the liquid strikes and cools the vane, there is the possibility of the occurrence of the erosion as described previously.
Further pertinent prior arts are described in U.S. Pat. Nos. 3,849,026; 3,856,423; and 3,936,227. They bear resemblance to this invention in point of the cooling of blades.